For disk devices such as a magnetic disk device and an optical disk device, which read/write data from/to a rotating recording disk with a head, the accurate positioning of the head on a target track is important for a higher recording density.
In the meantime, since the recording disk rotates, its position accuracy is deteriorated by a periodic disturbance such as eccentricity, etc. or other non-periodic disturbances. To implement a track pitch with high density, the position accuracy must be improved.
A head is positioned on a data track with a servo control in accordance with servo information recorded to a disk recording medium. Accordingly, the quality of the servo information, which is base information, exerts a significant influence on the positioning accuracy of a drive operation.
Specifically, for example, if a magnetic head is positioned on a target track of a magnetic disk in a hard disk device using a magnetic recording method, the magnetic head is moved toward the target track by controlling a voice coil motor in accordance with a difference between the current track position, which is determined based on the servo information that the magnetic head detects from the disk recording medium, and the target track position. When the magnetic head is determined to be moved to the target track position (enters an on-track state), the magnetic head is positioned at the center of the target track with a position signal obtained from the servo information. Then, data is written or read via the magnetic head.
Here, there is no information indicating a track position in a write of the servo information to the magnetic disk. Accordingly, the servo information is written, for example, by measuring the position of the magnetic head with laser ranging equipment, etc., and by positioning the magnetic head at a predetermined position on the magnetic disk. In this case, the phenomenon that the recording position of the servo information to be written deviates from an ideal center position of the track toward the radius direction of the magnetic disk due to influences of rotational fluctuations or vibrations of a spindle motor for rotating the magnetic disk, or the eccentricity of the magnetic disk, etc. occurs. This causes the phenomenon called RRO (Repeatable Run Out) that the actual center position of the track, which is detected at a read/write from/to the magnetic disk, fluctuates periodically, leading to a data read/write error, etc. caused by the overlapping of adjacent tracks.
For example, the technique using a post code recited in the following Patent Document 1 or 2 is known as the conventional technique for improving the RRO.
This conventional technique is described. Initially, a magnetic disk 701 includes a plurality of concentric tracks 702, each of which is partitioned, for example, into 108 regions 703 called a sector in the circumferential direction. Each sector 703 is partitioned into a region called a servo frame 704, and a data region 705 to which actual user data is recorded.
Each servo frame 704 is composed of a preamble 706, a servo mark 707, a gray code 708, four burst signals A, B, C and D 709, and a post code 710. The state of an actually recorded signal is represented as FIG. 2.
When the magnetic head is positioned on the target track of the magnetic disk, the servo frame 704 is detected by detecting the signal pattern of the preamble 706 while the magnetic head scans on the magnetic disk 701, subsequently the track number is obtained by detecting the gray code 708 after the servo mark 707 is detected, and the current track position is identified.
The voice coil motor is then controlled according to a difference between the current track position and the target track position, and a servo control for moving the magnetic head toward the target track is performed. Finally, the magnetic head is determined to have reached the target track position, namely, to have entered the on-track state at the time when the state where the current track position and the target track position match continues for a predetermined number of servo frames (such as eight frames).
Thereafter, the magnetic head detects the burst signals A to D 709 within the servo frame 704, whereby the magnetic head is positioned at the center of the track.
Here, in an inspection process after the magnetic disk 701 is mounted in the hard disk device, the magnetic disk 701 is rotated, its amount of eccentricity (=RRO) is measured for each sector 703, and the amount of correction for the eccentricity is written to the servo frame 704 (FIG. 1) of each sector as a post code 710.
When a read/write of the hard disk device is actually made, the current position including the eccentricity is calculated by reading and demodulating the burst signals 709 and the post code 710 from each servo frame 704 after the magnetic head 701 enters the on-track state, and by correcting the current position, which is obtained from the burst signals 709, with the amount of correction for the eccentricity of the post code 710, and the servo control of the head position that cancels the eccentricity is performed.
As schematically illustrated as FIG. 3, the recording positions of servo information 902 (=burst signals 709, etc.) to be written deviate from an ideal center position 901 toward the radius direction of the magnetic disk, which is indicated by 905, due to influences such as rotational fluctuations or vibrations of the spindle motor for rotating the magnetic disk, the eccentricity of the magnetic disk, or the like.
When such servo information 902 are read by the read head 904, the actual track center position, which is identified based on the servo information 902, includes an RRO component that fluctuates finely, for example, as represented by 1001 of FIG. 4A, leading to a read/write error.
In the meantime, since the value for correcting the eccentricity of the servo information 902 illustrated as FIG. 3 is recorded to the post code 903 illustrated as FIG. 3, a preferable position value 1003 with small fluctuations can be obtained by adding a post code value 102 to the position value 1001 before being corrected as represented as FIG. 4A.
FIG. 5A represents the enlarged view of this state. The position value 1001 with large fluctuations before being corrected as represented by 1101 is modified to the corrected control position value 1003 with small fluctuations as represented by 1102.
In the above described servo control using the post code, if the post code value 1002 to be corrected becomes an abnormal value as represented by 1104 due to some cause (for example, if the most significant bit is inverted) for the position value 1001 before being corrected, which is represented by 1103, the control position value 1003 after being corrected, which is modified with the abnormal value, also results in an abnormal value as represented by 1105, and can possibly cause an off track.
FIG. 4B is an example of actual waveforms. A large spike noise is superposed on the position value 1003 corrected with the post code value 1002, and an off track is caused since an abnormality occurs in the post code value 1002 as represented by 1004.
As the factor to cause an off track, for example, the lack of an offset margin of the read head 1203 due to an offset of the write position of the post code 1201 from the track center 1202 as illustrated as FIG. 6A is considered.
For example, also the lack of a read margin of the read head 1203 when a small defect exists adjacently to the post code 1201 as illustrated as FIG. 6B is considered.
In either case, a write cannot be made if the position of the magnetic head deviates from the target track due to an abnormal read of the post code value. Alternatively, the magnetic head is erroneously determined to be positioned at the center of the track although it is actually positioned at the border of the track with an abnormal post code value, and if a write is made in such a state, the write can be possibly made to an adjacent track, leading to a write off track.
The idea to add a parity bit to a post code and to correct the position with the post code if the result of the parity check is correct can be adopted. This method, however, includes the problems that the amount of information is increased by adding the parity bit, and a heavier load is imposed on the processing of the parity check.
[Patent Document 1] Japanese Laid-open Patent Publication No. 03-263662
[Patent Document 2] Japanese Patent Laid-open Publication No. 60-117461